Rotating piston & fluctuating power (LGU/LGV, TX?)

Discussion in 'Piston & Spring' started by Adam, Feb 28, 2019.

  1. Adam

    Adam Well-Known Member

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    Hi, I'm having an issue at the moment with power going up and down with my LGU. For example every few dozen shots I might get a few shots dropping out of kill at 50+ yds. Sometimes it's a couple, sometimes it's half a dozen. On a course this is not great as it's enough to start compensating by aiming high or adding clicks, only to have it go back to normal a few shots later and they start going high.

    I have a fairly speculative theory that it may be due to the rotating piston. If the comp tube is slightly out of round and the piston is able to rotate, there may be a little more friction at one spot. The piston moves round gradually and every so often a bearing hits a tighter spot, power drops, but then moves past and power goes back to normal.

    Fairly sure it's not the pellets as I've shot this batch of AA 8.4 for 18 months and they've been spot on. I'm going to clean the barrel and re-lead just to rule out a dirty barrel. I haven't ruled out lube yet either but I was curious if this is a thing.

    I wondered if this phenomenon had ever been noticed with other guns with rotating pistons. The other Walthers and the TX/PS.
     
  2. tillygti6

    tillygti6 Tilly's gun stocks

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    I cant help with the power issues, but I have seen a number of lgv and lgu with oval cylinders....
     
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  3. Adam

    Adam Well-Known Member

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    How much deviation?
     
  4. tillygti6

    tillygti6 Tilly's gun stocks

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    The worst was an lgv, it was clearly visible by eye, calipers around the flared part and iirc it was just over a mm larger od than the rest of the tube.
    I cant be sure if it was the same one , but one was visibly necked in at the rear edge of the cocking slot on the underside.

    Interestingly, all of the lgu/v I've had here so far have had the front fixings angles into the block different either side...
    Checked not cross threaded either..

    Hence why I dont get involved with them any more. It's a ball ache making up drilling jigs for each action.

    One of my customers over in Holland, found a few recent 97s that had ovalled tubes too.

    It took him a while to find one circular,to use for his build.
     
  5. Nick G

    Nick G Active Member

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    I have had this when developing tunes for the TX, I found that if your latch rod isn't concentric , as the piston rotates it rubs in the guide, causing inconsistency ,the piston turns one revolution in ten shots , so you should see the chrono change velocity as it rotates coming back to the same velocity after ten shots
     
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  6. RobF

    RobF Administrator Staff Member

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    Yeh but rotating pistons fix torque reaction :rolleyes: :D

    Could it be the piston seal heating/cooling or is it too fast for that?
     
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  7. Adam

    Adam Well-Known Member

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    So it gets hard to say because there is a temp effect at play too, which I believe is independent. But the switch between higher and lower shots has been too quick for purely temp.
     
  8. Adam

    Adam Well-Known Member

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    Thanks Nick, I’m going to check for that. I’ve not been able to nail down the frequency yet but if forced to guess, 20-30 shots for a cycle.
     
  9. RobF

    RobF Administrator Staff Member

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    You can get piston seals to warm up quite quickly and when that happens you can see the POI drop. So if it's going that way it might be, but if it's the opposite or not consistent then it might not be.
     
  10. Adam

    Adam Well-Known Member

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    Yeah that is a complicating factor I can't rule out entirely. There's always the possibility the whole thing may be a phantom effect of seal warming up and cooling down with tube expansion/contraction and lube viscosity, all happening at different rates. Bloody springers!
     
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  11. RobF

    RobF Administrator Staff Member

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    :D

    Yep. The seal is the normal culprit, they expand & contract way more than the comp tubes. We've measured it.

    When I was having issues I used to be able to shoot lines up and down at 25yds. The line would go up as the grease warmed and then go down as the piston seal expanded. If you're seeing it going randomly up and down then that might be something else. If it's trending then it might not be.

    I'll be zeroing my infernal pest tomorrow for Sunday's annual ritualistic equivalent of thrashing myself with a birch.
     
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  12. Adam

    Adam Well-Known Member

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    Annual? Not for me!
    Fortnightly! :eek::oops:
     
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  13. Adam

    Adam Well-Known Member

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    Rob, how did you fix the issues?
     
  14. RobF

    RobF Administrator Staff Member

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    Debatable that I did. I've got an o-ring piston in and I'll see how I go with that. I shot the last PJC with it and it seemed ok but it was freezing and I didn't take it that seriously and shot it in my snug down jacket instead. From what I remember it seemed ok, which considering I used to be able to see shift in my springers in most comps seemed to be progress. Looking back I think the reason why I might have seen it more than others is that I tend to shoot fast when I get going and so when I zero I probably have quite a relatively warm seal where as in the comps it's probably cooler.

    I got disheartened with springers having been through the mill several times.

    Dunno. I'm going to see what strategy helps tomorrow, I've got the whole day off. Assuming no gremlins have got in over the year it's not been shot I'm hoping it behaves and I can have a half decent bash at it. Never know if it's not a disaster then I might actually shoot it some more this year.
     
  15. Cooper_dan

    Cooper_dan Active Member

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    Fortnightly! Not for me!
    Weekly. Sometimes twice a week when we have saturday comps :D
     
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  16. Adam

    Adam Well-Known Member

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    Get help. Please.

    It's 2019, there's no stigma in asking these days. :D
     
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  17. Nomads HFT

    Nomads HFT Well-Known Member

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    Reading this thread earlier, I have been looking at the few records I have of the LGU recoil cycle, and found something unusual. The following has the caveat that it's based on just one example of the rifle, and may not affect other examples

    The surge of most spingers comprises a short accelerative period, followed by a longer period of relatively stable velocity, which is when the pellet exits. The LGU I tested seems to differ by having a longer accelerative period that persists beyond pellet exit.

    What this suggests is that the effect on POI of the tiniest change in pellet exit timing could potentially be more marked, and it wouldn't take much of a change in piston seal fit (possibly too little to cause much variation in muzzle velocity) to cause a POI shift by changing the pellet exit timing.

    I think it might be worth Adam's time running perhaps 50 pellets over a chrono, to see whether there is a drop in fps, and whether it's enough to cause his POI drop.
     
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  18. Cooper_dan

    Cooper_dan Active Member

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    I was going to chrono my lgu tomorrow so could do the same test. Any idea how long to wait between shots? Could easily set up a timer so the gap between shots is the same
     
  19. Nomads HFT

    Nomads HFT Well-Known Member

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    I don't know how rapidly FT shooters shoot, Dan.

    You could force the issue by putting ten shots over the chrono (with weighed pellets) as quickly as you can, see if that gets enough heat into the seal to make a difference - even a few fps alter the pellet exit point.
     
  20. RobF

    RobF Administrator Staff Member

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    I found when i was having issues that I could see shift in 20-30 pellets. So perhaps just shoot a string from cold and see what happens.

    What nailed it for us was when we took the comp tube and piston out and just measured their friction at different temps...
     

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